Field of the Invention
The invention relates to a circuit configuration and a method for setting the switching points of a decision maker which is actuated by an analog input signal.
Sensors are used in a known manner to detect the movement or the position or orientation state of rotating parts. Examples of this include crankshaft, cam shaft, transmission and ABS sensors in automobiles. Hall sensors are preferably used as sensors and monitor a change in a magnetic field. To this end, for example, a permanent magnet is fitted to a stationary part, in order to produce a magnetic field. The magnetic field is then modulated by a gear wheel mounted on the rotating part or by some other ferromagnetic transmitter, depending on the position or orientation. The Hall sensor is in this case preferably located between the permanent magnet and the gear wheel or transmitter, and can thus detect fluctuations in the magnetic field. If, for example, a tooth of the gear wheel is located in the magnetic field, then a xe2x80x9chighxe2x80x9d output signal is supplied, while a gap between the teeth results in a xe2x80x9clowxe2x80x9d output signal. In this way, the signal emitted from the Hall sensor can be used to deduce the instantaneous orientation or position of a rotating part.
The signal supplied from a sensor is influenced substantially by the operating conditions in which the sensor is used. These operating conditions include unavoidable incalculable factors, such as the operating temperature or the size of the air gap etc. Despite the fluctuations caused by the operating conditions, the sensor should supply an output signal that is as well defined as possible. Therefore, the output signal should have a well-defined profile irrespective of fluctuations caused by the operating conditions. The reason for this is now described. If, for example, a sensor configuration supplies a sinusoidal signal, then a system controlled by the sensor configuration can maintain a well-defined response when the system switching processes, which depend on the output signal from the sensor, are carried out at the zero crossings of the signals. Specifically, the zero crossings are independent of the respective signal amplitude and, furthermore, have a high edge gradient.
If the output signal from the sensor has other signal forms, a switching point other than the zero crossing or signal mid-point may, of course, also be advantageous.
When evaluating the output signal of a sensor for switching a system which is controlled by this sensor, a switching point should therefore be maintained irrespective of the signal amplitude of the output signal from the sensor, and this applies even to very slow signals. In detail, VDI Reports 1287, 1996, pages 583 to 611, titled xe2x80x9cEine neue Generation von xe2x80x9cHall-Effektxe2x80x9d-Zahnradsensoren: Vorteile durch die Verbindung von BIMOS-Technologie und neuen Verpackungsrezeptenxe2x80x9d [A new Generation of xe2x80x9cHall Effectxe2x80x9d Gearwheel Sensors: Advantages Resulting From the Use of BIMOS Technology and New Packaging Forms] describes a sensor configuration in which the amplitude of the output signal from the sensor is initially normalized, possibly with the aid of an analog/digital converter. The signal peak values are detected with the aid of two further analog/digital converters and digital/analog converters. This is used to derive and define a switching threshold. In the end, this allows a system response to be achieved which is essentially independent of temperature fluctuations and the width of the air gap. However, the complexity required for the sensor configuration is relatively high, since gain matching and numerous analog/digital converters are required.
It is accordingly an object of the invention to provide a circuit configuration and a method for setting the switching points of a decision maker which overcomes the above-mentioned disadvantages of the prior art devices and methods of this general type, which is actuated by an analog input signal, in which configuration and method switching processes can be set reliably and with high accuracy at selected points of an input signal, with little complexity.
With the foregoing and other objects in view there is provided, in accordance with the invention, a circuit configuration for correcting an analog input signal having a DC element, an AC element, and upper and lower signal peaks whose ratio to one another is fixed but variable. The analog input signal being corrected by compensation for the DC element. The circuit configuration contains a decision maker receiving the input signal and a calibration device. The calibration device includes an analog/digital converter actuated by the input signal and is connected to the decision maker. Peak-value detectors are provided for determining the upper and lower signal peaks of the input signal, the peak-value detectors are disposed downstream from the analog/digital converter. A controllable reference unit is provided for generating a reference value. A computation unit for determining a mean value from at least one signal minimum and at least one signal maximum is disposed downstream from the peak-value detectors. The mean value is either an arithmetic mean of the upper and lower signal peaks or mean weighted using the fixed but variable ratio. A comparison unit for determining a signal position of the input signal by comparing the reference value with the mean value determined by the computation unit is provided. The comparison unit is connected to the computation unit and to the controllable reference unit. A first regulating unit has an input connected to the comparison unit and an output linked to the decision maker. The first regulating unit compensates for the DC element in the input signal if an unbalanced signal position is detected by the comparison unit, with the compensation being carried out by subtraction of a value which is determined from an output of the comparison unit from the input signal. A second regulating unit has an input connected to the comparison unit and an output connected to the controllable reference unit. The second regulating unit readjusts the reference value in an opposite sense to an output signal from the comparison unit, in which case a difference between the mean value and the reference value is used to form a new reference value. A device for scaling the mean value is disposed between the computation unit and the comparison unit or an output side of the comparison unit.
The circuit configuration according to the invention for setting the switching points of a decision maker which is actuated by an analog input signal, independently of any DC element which is included in the input signal and together with upper and lower signal peaks, contains a control device having peak-value detectors. The peak-value detectors are actuated by the input signal, for determining the upper and lower signal peaks of the input signal. Furthermore, the control device has a controllable reference unit for providing a reference value, a computation unit connected downstream from the reference unit and the peak-value detectors for determining the mid-value from at least one upper and one lower signal peak, and a comparison unit for determining the signal position of the input signal by comparison of the reference value with the mid-value determined by the computation unit. Finally, a first regulating unit is provided whose input side is connected downstream from the comparison unit and whose output side is linked to the decision maker, in order to compensate for the DC element in the input signal by a compensation signal when a DC element is detected by the comparison unit.
A second regulating unit, whose input side is connected downstream from the comparison unit and whose output side is connected to the reference unit, is preferably also provided, for readjusting the reference value in the opposite sense.
The circuit configuration according to the invention allows the DC element contained in an analog input signal, independently of an AC element, to be determined with high accuracy and with relatively little complexity, and this is then used to set the switching threshold of the decision maker. It is particularly advantageous for the signal path on which the input signal is routed to contain virtually only the decision maker itself, so that the signal path is configured to be as simple as possible and thus does not itself cause any significant adverse effect on the input signal.
The control device preferably has circuitry for digital signal processing. In this case, an analog/digital converter is connected upstream of the control device, and a digital/analog converter is connected downstream from it. The digital signal processing offers high-precision signal processing with little complexity.
In one development of the invention, a controllable amplifier is connected upstream of the control device. The controlled input of the amplifier is in this case connected to a control output of the control device in order to control the gain of the amplifier as a function of the instantaneous value of the signal fed into the control device. The controllable amplifier in conjunction with the analog/digital converter results in an analog/digital converter with a logarithmic response, which allows very fine resolution in the region of the decision-maker switching point. It is thus even possible to use analog/digital converters that have only low resolution, but require only a low level of circuitry complexity. High accuracy can thus nevertheless be achieved with low circuitry complexity.
The amplifier gain preferably has binary steps and can be controlled by binary words. Binary stepping has the advantage that logarithmic gain changes can be achieved easily by shifting a binary number.
Finally, the control device may have a monitoring device for the decision maker and a timer. The output of the decision maker is connected to the control device and the control device monitors the output of the decision maker for signal changes, and initiates determination of the DC element if there is no signal change within a time governed by the timer. This prevents very slow changes in the DC element from not being detected by the configuration.
The invention also relates to a method for setting switching points for the decision maker. The decision maker is actuated by the analog input signal having a DC element which is contained in the input signal in addition to an AC element.
The method provides for determining an instantaneous signal profile of the input signal, preferably the time profile of the amplitude of the input signal. The lower and upper signal peaks in the signal profile are then determined. A mean value is calculated from at least one lower and upper signal peak. A discrepancy between the mean value and the predetermined reference value, which reflects the DC element, is calculated. The discrepancy is subtractively linked to the input signal. In this case, it is also possible to provide for the switching point to be set continuously, without specific external influences or signals initiating a setting process.
In a development of the invention, the instantaneous signal profile of the input signal is determined. At least the lower and upper signal peaks in the signal profile are determined. The mean value is calculated from the signal peaks and a discrepancy between the mean value and a predetermined reference value is calculated. The discrepancy is compared with a tolerance threshold value and, if the magnitude of the discrepancy exceeds the tolerance threshold value, it is subtractively linked to the input signal. Therefore, relatively small changes in the DC element are ignored, as a result of which, for example, relatively minor interference or noise has no influence on the setting of the switching point.
Furthermore, the instantaneous signal profile of the input signal can be determined, at least the upper and lower signal peaks in the signal profile can be determined, and the mean value can be calculated from these signal peaks. The discrepancy between the mean value and a predetermined reference value can be calculated, and the discrepancy can be compared with zero. If the discrepancy is greater than zero, a specific constant value can be subtractively linked to the input signal and, if the discrepancy is less than zero, can be additively linked to the input signal. Therefore, the switching point is varied only in fixed steps.
Alternatively, the instantaneous signal profile of the input signal can be determined, at least one lower and upper signal peak can be determined in the signal profile, and the mean value can be calculated from these signal peaks. The discrepancy between the mean value and a predetermined reference value can be calculated. The discrepancy can be compared with zero and the discrepancy can be allocated on the basis of its magnitude to one of three ranges for large, medium and small discrepancies and, depending on the range to which the discrepancy has been allocated, either the discrepancy, a constant value or nothing can be subtractively linked.
Furthermore, the respective instantaneous signal profile of the input signal can be determined continuously. At least one lower and upper signal peak in the respective signal profile can be determined. The mean value can be calculated from these signal peaks. The discrepancy between the mean value and a predetermined reference value can be calculated, and the discrepancy can be compared with zero. The discrepancy is further compared with a high and a low threshold value and, if the low threshold value is undershot, the input signal can remain unchanged. If the low threshold value is exceeded, a constant value can be continuously additively linked to the input signal if the discrepancy is less than zero, or can be subtractively linked if the discrepancy is greater than zero and, if the high threshold value is exceeded, the discrepancy can be continuously subtractively linked to the input signal until the low threshold value is undershot once again.
Furthermore, the output signal from the decision maker can be monitored for signal changes, and renewed determination of the DC element can be initiated if there is no signal change within a specific time. This makes it possible to avoid a slowly changing DC element not being identified.
Furthermore, in order to improve the interference immunity, the output of the decision maker can be switched off during the setting process. It is furthermore possible to provide for relative extremes in the signal profile to be assumed to be peak values only when the signal profile changes by a specific value at the extremes. This prevents, for example, relatively small relative extremes caused by interference being identified as signal peaks.
Furthermore, it is possible for first signal peaks not to be evaluated and only the subsequent signal peak or peaks to be used for evaluation. This measure likewise improves the interference immunity.
In order to achieve better resolution, in particular in conjunction with low-resolution analog/digital converters, the signal profile can be amplified before the signal peaks are determined, in such a manner that the gain is initially high and, if overdriving occurs, the gain is reduced until overdriving no longer occurs.
It is furthermore possible to provide for the signal amplitude to be determined and to be compared with an amplitude threshold value and, if the gain is relatively high, for the signal to be amplified when the signal amplitude falls below a specific value.
Finally, in one development of the invention, the mean value formed from the signal peaks can be a xe2x80x9cweighted meanxe2x80x9d which is calculated using the formula       M    =                            x          xc3x97          A                +                  y          xc3x97          B                            x        +        y              ,
with A being the signal maximum, B being the signal minimum and x, y being element factors. If, for example, x=y=1, then this is an arithmetic mean. In this case, the maximum value has the same influence as the minimum value on the correction value. Correction using a weighted mean is particularly worthwhile for encoder wheels whose output signal has an asymmetric profile (input to the decision maker).
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a circuit configuration and a method for setting the switching points of a decision maker, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.